Sorry, I've done way too much physics/math tutoring to let this slide: "miles per passenger per gallon of fuel" puts the number of passengers in the denominator with the amount of fuel consumed. The correct unit is used in the parentheses: "passenger-miles per gallon" which puts the number of passengers in the numerator, where it belongs (more miles or more passengers -> higher # -> good; more fuel -> lower # -> bad)

Pedantic much? How about (miles/passenger)/gallon? Looks like miles per passenger per gallon to me. Passenger-miles just sounds like an obscurant euphemism.

But its not -- (miles/passenger)/gallon

its -- (milesXpassenger)/gallon